DIGESTION 317 



HI. The Secretion of the Digestive Juices. 



The digestive glands are formed originally from involu- 

 tions of the mucous membrane of the alimentary canal, the 

 salivary glands from the epiblast, the others from the hypo- 

 blast (Chap. XIV.). Some are simple unbranched tubes, in 

 which there is either no distinction into body and duct, as 

 in Lieberkiihn's crypts in the intestines, or in which one or 

 more of the tubes open into a duct, as in the glands of the 

 cardiac end of the stomach. Some are branched tubes, 

 several of which may end in a common duct ; such are the 

 glands of the pyloric end of the stomach, and the Brunner's 

 glands in the duodenum. In others the main duct ramifies 

 into a more or less complex system of small channels, into 

 each of the ultimate branches of which one or more (usually 

 several) of the secreting tubules or alveoli open. The 

 salivary glands and the pancreas belong to this class of 

 compound tubular or racemose glands, and so does the liver 

 of such animals as the frog. But in the latter organ the 

 typical arrangement is obscured in the higher vertebrates by 

 the predominance of the portal bloodvessels over the system 

 of bile-channels as a groundwork for the grouping of the cells. 



In every secreting gland there is a vascular plexus outside 

 the cells of the gland -tubes, and a system of collecting 

 channels on their inner surface ; and in a certain sense the 

 cells of every gland are arranged with reference to the blood- 

 vessels on the one hand, and the ducts on the other. But 

 in the ordinary racemose glands the blood-supply is mainly 

 required to feed the secretion ; the cells of the alveoli have 

 either no other function than to secrete, or if they have other 

 functions, they are not such as to entail a great disproportion 

 between the size of the cells and the lumen of the channels 

 into which they pour their products. For both reasons the 

 relation of the grouping of the cells to the duct-system is 

 very obvious, to the blood-system very obscure. In the liver 

 the conditions are precisely reversed. We cannot suppose 

 that the manufacture of a quantity of bile less in volume 

 than the secretion of the salivary glands, though doubtless 

 containing far more solids, requires an immense organ like 



